Synthesis gas mixtures essentially comprising carbon monoxide and hydrogen are important commercially as a source of hydrogen for hydrogenation reactions, and as a source of feed gas for the synthesis of hydrocarbons, oxygen-containing organic compounds, and ammonia. One method of producing synthesis gas is by the gasification of coal, which involves the partial combustion of this sulfur-containing hydrocarbon fuel with oxygen-enriched air. In the slagging-type gasifier, a coal-water slurry and oxygen are used as fuel. These two streams are fed to the gasifier through a feed injector, sometimes called a burner, that is inserted in the top of the refractory-lined reaction chamber. The feed injector uses two oxygen streams and one coal slurry stream, all concentric, which are fed into the reaction chamber through a water-cooled head. The reaction chamber is operated at much higher pressure than the injector water jacket.
In this process, the reaction components are sprayed under significant pressure, such as about 80 bar, into the synthesis gas combustion chamber. A hot gas stream is produced in the combustion chamber at a temperature in the range of about 700° C. to about 2500° C., and at a pressure in the range of about 1 to about 300 atmospheres, and more particularly, about 10 to about 100 atmospheres. The effluent raw gas stream from the gas generator typically includes hydrogen, carbon monoxide, and carbon dioxide, and can additionally include methane, hydrogen sulfide, and nitrogen, depending on fuel source and reaction conditions.
This partial combustion of sulfur-containing hydrocarbon fuels with oxygen-enriched air presents problems not normally encountered in the burner art. It is necessary, for example, to effect very rapid and complete mixing of the reactants, as well as to take special precautions to protect the burner or mixer from overheating. Because of the tendency for the oxygen and sulfur contaminants in coal to react with the metal from which a suitable burner may be fabricated, it is necessary to prevent the burner elements from reaching temperatures at which rapid oxidation and corrosion takes place. It is therefore essential that the reaction between the hydrocarbon and oxygen take place entirely outside the burner proper, and that the localized concentration of combustible mixtures at or near the surfaces of the burner elements be prevented.
Even though the reaction takes place beyond the point of discharge from the burner, the burner elements are subject to radiative heating from the combustion zone, and by turbulent recirculation of the burning gases. For these and other reasons, the burners are subject to failure due to metal corrosion about the burner tips, even though these elements are water-cooled, and though the reactants are premixed and ejected from the burner at rates of flow in excess of the rate of flame propagation. Typically, after a short period of operation, thermal corrosion fatigue cracks develop in the part of the jacket that faces the reaction chamber. Eventually these cracks penetrate the jacket allowing process gas to leak into the cooling water stream. When leaks occur, gasifier operation must be terminated to replace the feed injector.
Attempts have been made in the past, with varying levels of success, to minimize this resulting corrosion. For example, U.S. Pat. No. 5,273,212 discloses a shielded burner clad with individual ceramic tiles, or platelets, arranged adjacent each other so as to cover the burner in the manner of a mosaic.
U.S. Pat. Nos. 5,934,206 and 6,152,052 describe multiple shield segments attached to the face of the feed injector by brazing. These shield segments are typically ceramic tiles, though other high melting point materials can also be used. Each of these tiles forms an angular segment of a tile annulus around the nozzle, the tiles being overlapped at the radial joints to form stepped, or scarfed, lap joints. The individual tiles are secured to the coolant jacket end face by a high temperature brazing compound.
U.S. Pat. No. 5,954,491 describes a wire-locked shield face for a burner nozzle. In this patent, a single piece ceramic heat shield is attached to the feed injector by passing high temperature alloy wires through the shield and a series of interlocking tabs. The shield is thus mechanically secured over the water jacket end-face of the injector nozzle, and is formed as an integral ring or annulus around the nozzle orifice.
U.S. Pat. No. 5,947,716 describes a breech lock heat shield face for a burner nozzle. The heat shield is comprised of an inner and an outer ring, each of which forms a full annulus about the nozzle axis, shielding only a radial portion of the entire water jacket face. The inner ring is mechanically secured to the metallic nozzle structure by meshing with lugs projecting from the external cone surface of the nozzle lip. The internal perimeter of the inner ring is formed with a channel having a number of cuts equal to the number of lugs provided, so as to receive the respective external lug element. When assembled, the inner ring is secured against rotation by a spot-welded rod of metal applied to the nozzle cooling jacket face within a notch in the outer perimeter of the inner ring.
The outer perimeter of the inner ring is formed with a step ledge, or lap, approximately half the total thickness of the ring, that overlaps a corresponding step ledge on the internal perimeter of the outer ring. The outer ring is also secured to the water jacket face by a set of external lug elements, projecting from the outer perimeter of the water jacket face. A cuff bracket around the perimeter of the outer ring provides a structural channel for receiving the outer set of water jacket lugs. The outer heat shield ring is also held in place by a tack-welded rod or bar.
U.S. Pat. No. 5,941,459 describes a fuel injector nozzle with an annular refractory insert interlocked with the nozzle at the downstream end, proximate the nozzle outlet. A recess formed in the downstream end of the fuel injector nozzle accommodates the annular refractory insert.
U.S. Pat. No. 6,010,330 describes a burner nozzle having a faired lip protuberance, a modification to the shape of the burner face that alters the flow of process gas in the vicinity of the face. This modification results in improved feed injector life. A smooth transition of recirculated gas flow across the nozzle face into the reactive material discharge column is believed to promote a static or laminar flowing boundary layer of cooled gas that insulates the nozzle face, to some extent, from the emissive heat of the combustion reaction.
U.S. Pat. No. 6,284,324 describes a coating that can be applied to the shields previously described, to thereby reduce high temperature corrosion of the shield material.
U.S. Pat. No. 6,358,041, the disclosure of which is incorporated herein by reference, describes a threaded heat shield for a burner nozzle face. The heat shield is attached to the feed injector by means of a threaded projection that engages a threaded recess machined in the back of the shield. The threaded projection can be a continuous member or a plurality of spaced-apart, individual members provided with at least one arcuate surface. This threaded method of attachment is a reliable way to attach the heat shield to the feed injector. It provides greater strength, and is more easily fabricated than other shield attachments. This is especially true when the shield is made of a metal that is easily machined.
Although the heat shield just described is a significant advance in the art, permitting extended operation times, the operational life is nonetheless limited by the corrosion that occurs at the center of the shield. Operating experience using the threaded attachment method has revealed that a local zone of high oxygen activity causes corrosion of the molybdenum shield. This local zone of high oxygen activity is caused by the gas flow dynamics of the oxygen stream as it exits the feed injector. An area of low pressure exists just outside the lip on the face of the injector. This low pressure zone draws in oxygen, causing corrosion of the molybdenum shield.
While molybdenum has extremely good resistance to corrosion by reducing gases, it is not so resistant to high temperature oxidation. As the shield corrodes, the protection it provides to the face of the injector is gradually lost, shortening the life of the injector. When this occurs, corrosion of both the back of the shield and the face of the injector results. This corrosion is particularly severe at the base of the threaded attachment ring that protrudes from the face of the injector. In extreme cases, the corrosion has been known to cause the threaded ring to fail and the shield to depart.
Although the addition of a coated molybdenum shield to the face of the feed injector has doubled the maximum run length of the feed injector, run length is still limited by oxidation of the shield which occurs near the center of the shield, leading to corrosion and cracking of the injector face. As the condition of the shield further deteriorates, more corrosive material accumulates between the shield and the injector face. This causes failure of the attachment ring, and eventual loss of the shield.
There remains a need to provide a heat shield design for a burner for synthesis gas generation which is an improvement over the shortcomings of the prior art in terms of operational life expectancy, which is simple in construction, and which is economical in operation.
It is therefore an object of the invention to further extend the operational life expectancy of the gas generation burner nozzle just described.
Another object of the invention is to provide a gas generation burner nozzle for synthesis gas generation having a reduced rate of corrosion.
A further object is to provide a burner nozzle heat shield to protect metallic elements of the nozzle from the effects of corrosion caused by combustion gases.
Yet another object is to provide a ceramic insert that is specifically resistant to the effects of oxygen in removing the molybdenum from the oxidizing zone, thereby protecting the threads that attach the shield to the injector from the effects of corrosion caused by combustion gases.